I think these modern tiles are based on the original 1900's ones from when this early stretch of the Northern Line was the Hampstead Tube, and they were put up in a mid-'90s refurbishment. Right now I'm looking at a photo of Euston in 1907, which I'll try & scan later, and it looks the same. I don't think they have them anywhere at Euston now, but have you seen them on any other line apart from the stretch of the Northern between Hampstead & Mornington Crescent.

I actually found this image online, but I was intrigued by it.

I keep thinking I've seen it on some of the more central stations, but can't recall any right now.

Thanks for your help!

In case anyone knows a good match, here are the original tiles at Euston in 1907:

And my photos of the new tiles which were at the earliest mid-'90s.

London Transport seem to be very good at archiving all their material, and as the original plans for the 1900s Hampstead Tube stations still exist I expect they had the drawings of the original lettering to work from.